The Code Division Multiple Access (CDMA) standard (IS-95) for cellular communication systems calls for an 8 Kbps voice coding (vocoding) algorithm. A vocoder converts speech into a digital data stream. A number of techniques have been used to reduce the data rate necessary to send acceptable speech signals. Despite this any given voice coding system produces a higher quality speech signal when it has a higher data rate.
Field tests of the 8 Kbps vocoder for the CDMA cellular systems have shown non-optimal speech quality. The obvious solution is to increase the data rate of the vocoder. This solution can have major impacts on other parts of the CDMA cellular telephone system.
Part of a CDMA cellular system 10 is shown in FIG. 1. A base station controller (BSC) 12 is coupled to a number of base transceiver sites 14 (BTS) through T1 telephone lines 16. The BTS 14 is in radio communication with a plurality of mobiles (portables or subscriber units) 18. In the CDMA system 10 incoming voice data is received by the BSC 12 from the public switched telephone network (PSTN) 20. A single voice channel consists of a 64 Kbps, pulse code modulation (PCM) signal. The BTS 14 transmits a single voice channel at 16 Kbps, including overhead data. The BSC 12 has a plurality of transcoders (XCDR) 22 that compress the 64 Kbps voice channels from the PSTN into 16 Kbps voice and control channels. Since the BSC 12 has more than a single channel of voice coming in from the PSTN 20 a demultiplexer 24 is necessary to separate the channels. Once the channels have been transcoded, they are multiplexed, by multiplexer 26 onto the T1 line 16. A controller 28 controls all aspects of the BSC 12.
The BTS 14 receives the plurality of voice signals from the T1 line 16 and demultiplexer 30 separates the voice channels and directs them to one of a plurality of transceivers (XCVR) 32. The transceivers 32 modulate the received voice signals for radio transmission over an antenna 34. A combiner/separator 36 couples the plurality of transceivers 32 to the antenna 34. The BTS 14 has a global positioning system (GPS) receiver 38, the function of which will be explained more fully below. A controller 40 coordinates and controls all aspects of the BTS 14.
The received voice signals from the PSTN 20 do not contain any of the control information necessary for the BSC 12 to communicate with the BTS 14. This control information is added by the BSC 12. The control information and transcoded voice data are transmitted in a frame. An example of a prior art frame 50 is shown in FIG. 2. The prior art frame 50 contains 320 bits and is 20 ms long. The frame 50 has a 35 bit synchronization word, 37 bits of control data, 202 bits of data (voice), 42 bits for a Universal Time Counter (UTC) and 4 T bits. In the prior art 8 Kbps vocoder only 166 of the 202 bits were used for the voice signal. Any vocoder changes to the CDMA standard need to fit within a 16 Kbps frame unless a major redesign of other parts of the architecture is undertaken. To meet the desired voice quality standards a 13.25 Kbps vocoder was chosen. This requires 265 data bits per frame.
A new frame structure was devised to achieve the 13.25 Kbps vocoding scheme. First, all the available data bits are used, increasing the available data bits to 202. The UTC bits are only sent on non-full rate links, freeing up 42 bits. The Hamming code portion of the control data bits are eliminated as unnecessary, freeing up 5 bits. The bad frame quality indicator is reduced to 9 bits. However, the frame sequence bits have to be increase by 1 bit. All these changes allow 253 bit per frame, twelve bits short of the necessary 265 bits.
The 4 T bits were rarely used, and there only function is to reserve time in case a frame needs to be advance in time. Further the frame and time alignment bits were not needed every frame, but were expected by the BTS 14 and BSC 12. Thus there exists a need for a frame and time alignment system and method that uses fewer bit per frame to free up bits for data.